1. Field of the Invention
This invention relates to a pan coating apparatus, and more particularly to improvements in a pan coating apparatus for coating particles such as tablets and granules of pharmaceuticals, foods, and the like.
2. Description of the Prior Art
As an apparatus for coating particles such as tablets and granules, at present, generally an apparatus disclosed in Japanese Patent Publication No. 50-38713 and improved apparatuses based on 50-38713 have been used.
All of these conventional apparatuses are constructed such that a rotatable container (coating pan), in which at least a part of side walls is formed of a perforated plate, is rotated about a horizontal axis; coating solution is sprayed from a spraying device; air is introduced into the coating pan for drying coating solution applied to the surfaces of particles; and air is exhausted from an inside space of the coating pan, passing through an accumulated particle layer, the perforated plate and an air duct formed to cover the perforated plate portion of the coating pan.
The most important differences among the apparatus disclosed in 50-38713 and its improved version reside in methods of introducing air.
As shown in FIG. 11, in the apparatus of 50-38713 as being a prototype, an opening is provided in a container on the side surface vertical to an axis B of rotation of the coating pan A, and particles are charged through an inlet opening of a fixed cylinder provided in a manner that the periphery of the opening of the container is slidable on this cylinder. The introduction of air is performed through an air supply duct provided on the side surface of the fixed cylinder, and the air is exhausted through perforations C provided in the peripheral surface of the container.
An apparatus described in Japanese Unexamined Patent Application Laid-Open No. 60-500894 (Refer to FIG. 12(a)) is similar in type to the above-described apparatus except that out of perforated plates provided on the side surface of a container, air is introduced through a perforated plate D positioned at the top and substantially opposed to an accumulated particle layer, and exhausted through a perforated plate E positioned at the bottom, being in contact with the bottom portion of the accumulated particle layer. An air distributor F (Refer to FIG. 12(b)) is provided for this purpose.
An apparatus described in Japanese Unexamined Patent Application Laid-Open No. 2-207833 (Refer to FIG. 13) is of such an arrangement that an air pipe G is provided to project into a coating pan A in a manner that the base portion of the pipe is slidable with the vertical surface of the container; air is blown out of a cut-away portion of the air pipe G into the coating pan A; and the direction of blow-out is variable.
Furthermore, in these conventional apparatuses, direction of the air flow may be reversed in such a way that air is introduced from a perforated plate of the coating pan, which is positioned in contact with the bottom portion of the accumulated particle layer, and the air is exhausted through the above-described various air introducing paths.
However, in the conventional apparatuses using the above-described various air introducing methods, there are following disadvantages, respectively.
Firstly, in the apparatus described in 50-38713, air flows from the front of the coating pan (in the left in FIG. 11) to the rear and downward, and particularly, when an air supply pipe is narrow at a portion where air is introduced through the opening of the coating pan, the flow velocity of the introduced air is high, whereby sprayed liquid drops are caused to drift and a spray pattern is disturbed, and a coating layer on a particle tends to be non-uniform.
If the cross-sectional area of the introducing portion is increased, then the flow velocity becomes lower and the disturbance of the spray pattern becomes small. In that case, however, air flow passes mainly through the front portion of the pan while poorly passing through the accumulated particle layer in the rear portion of the pan, so that the drying efficiency is decreased.
In the apparatus described in 60-500894, air is introduced through the perforated plate D positioned at the top of the pan, the air flows slowly from above to below over the whole area in the pan and there is such an advantage that the spray pattern is not disturbed. However, when a duct is positioned at the top, the duct is used for introducing air, and, when the same duct is positioned at the bottom, the duct is used for exhausting air, whereby dried particles of spray mist and dust produced due to abrasion of the particles enter the duct at the time of exhausting air, and, when this duct is turned to the top, the dust is blown back into the pan, so that the dust adheres to the surfaces of the particles, thus leading to rough coating surfaces.
Furthermore, when one and the same coating pan is used for coating particles of different kinds, the particles which have been coated in the former batch and dust produced from the dried coating solution remain in the ducts and adhere to the particles, so that fatal disadvantages are presented from the view point of good manufacturing practice (GMP) when, for example, the particles to be processed are pharmaceuticals.
In the apparatus described in 2-207833, although there are differences depending on the position of an air pipe G projected into the pan, the size of a cutaway portion H for blowing out air, the direction of blow-out and so forth, the air flow velocity is high at the portion where air is blown out, so that the spray pattern becomes disturbed.
Furthermore, the air flow velocity of the air stream is fairly high within the air pipe, whereby, even if the blow-out opening is directed downward, air is blown to the front side of the pan (left in FIG. 13), so that coating solution tends to adhere to the inner wall of the pan and there may occur a dusting phenomenon in which the liquid drops of the coating solution is dried to become fine powder before the liquid drops adhere the particles.
Although this diadvantage may be lessened by increasing the diameter of the air pipe, the diameter of the air pipe cannot be increased to the extent where this disadvantages can be completely obviated, because of the construction of the pan.
In all of the above-described apparatus, introduction of air becomes same when the air flow is reversed. In this case, air flow becomes reverse to the blow-out direction of the spray solution, so that the spray pattern is disturbed extremely and the above-described dusting tends to occur.
Thus, there are disadvantages in all of the conventional methods of introducing air into the apparatuses, and no satisfactory method has been found.
It is an object of the present invention to provide a pan coating apparatus having a gas blow-out portion capable of forming and maintaining the satisfactory spray pattern.